Remove EncryptionInterstitial for Full Disk Encryption

Support for Full Disk Encryption was removed in Android 13, since now
File Based Encryption is always used instead.  It turns out that I
missed a fairly large chunk of obsolete code: EncryptionInterstitial,
which is the screen that asks whether the device will require the
primary user's lockscreen credential when it starts up.  This used to be
shown when setting the primary user's lockscreen credential, to
determine whether the full-disk encryption key would be tied to that
lockscreen credential or not.  But now it's unused code.

This CL removes all this unused code.

This should not change any behavior, with one very minor exception:
Settings will no longer explicitly set the REQUIRE_PASSWORD_TO_DECRYPT
setting to 0 whenever the primary user's lockscreen credential is
changed.  (This happened in SaveChosenLockWorkerBase.)  This setting is
a @SystemApi, but it no longer has any meaning, since it is never set to
1 anymore.  If there is a reason to keep it explicitly set to 0, instead
of unset, we should make LockSettingsService in system_server set it.

Test: Went through SUW, set a PIN, cleared the PIN, set a PIN again (all
      using the UI).  Nothing unusual seen.
Bug: 208476087
Change-Id: I039cc7a284e3f43e1e284970a5869958c909d1b7